1. Field of the Invention
The present invention relates to an active matrix organic light emitting display (AM-OLED) with an amorphous silicon thin film transistor (a-Si:H TFT) as a driving device and, more particularly, to an AM-OLED with a light-shielding structure that isolates parasitic OLEDs outside a display area and prevents damage to an amorphous silicon layer from subsequent surface treatments.
2. Description of the Related Art
In accordance with driving methods, an organic light emitting display (OLED) can be an active matrix type or a positive matrix type. The active matrix organic light emitting display (AM-OLED) is driven by electric currents, in which each of the matrix-array pixel areas has at least one thin film transistor (TFT), serving as a switch, to modulate the driving current based on the variation of capacitor storage potential so as to control the brightness and gray level of the pixel areas. At present, the AM-OLED is driven by two TFTs in each pixel area, and, alternatively, the AM-OLED is driven by four TFTs in each pixel area.
Concerned with the luminescent principle of the AM-OLED, an electric current is applied to a specific organic lamination to change electricity into luminescence. The AM-OLED has panel luminescence with thin-type and light-weight characteristics, spontaneous luminescence with high luminescent efficiency and low driving voltage, and advantages of view angle, high contrast, high-response speed, full color and flexibility. As for the fabrication of the TFTs in the AM-OLED, an amorphous silicon (a-Si:H) TFT process that has been popularly applied to the fabrication of large-size liquid crystal displays (LCDs) is integrated into the AM-OLED process.
Use of two a-Si:H TFTs in each pixel area is an example to describe the conventional a-Si:H TFT process. FIG. 1 is a top view showing an a-Si:H TFT of an AM-OLED according to prior art. The AM-OLED comprises a plurality of pixel areas 10 arranged in a matrix form that are constituted by a plurality of data lines 12 extending along a Y direction and a plurality of source lines (also called Vdd lines) 14 extending along an X direction. Also, each pixel area 10 comprises two scanning lines 16 extending along the X direction, two a-Si:H TFTs 18 respectively disposed over the two scanning lines 16, a pixel electrode 20 of rectangular-shaped transparent conductive material disposed between the two scanning lines 16, and a capacitor 22.
In general, the a-Si:H TFT process can be an etching stopper type and a back channel type. Hereinafter, use of the etching stopper type is an example to describe the a-Si:H TFT process of the prior art. FIG. 2A is a sectional diagram along line A–A′ of FIG. 1 to show the a-Si:H TFT 18. First, a first metal layer is deposited on a transparent substrate 30 and then patterned as the source line 14, the scanning line 16 and a bottom electrode of the capacitor 22. Next, a first insulating layer 32, a second insulating layer 34 and an a-Si:H layer 36 are successively deposited on the entire surface of the transparent substrate 30. Then, using photolithography and etching to remove parts of the second insulating layer 34 and the a-Si:H layer 36, an island structure is patterned over the predetermined area of the a-Si:H TFT 18. Also, the second insulating layer 34 and the a-Si:H layer 36 disposed over the source line 14 are completely removed. Thereafter, an etch stopper 38 is formed on the island structure over the predetermined area of a gate electrode. After a doped amorphous silicon layer 40 and a second metal layer 42 are successively deposited on the entire surface of the transparent substrate 30, photolithography and etching are used to remove pars of the doped amorphous silicon layer 40 and the second metal layer 42, thus the second metal layer 42 is patterned as the data line 12 and an upper electrode of the capacitor 22. It is noted that the doped amorphous silicon layer 40 and the second metal layer 42 disposed on the island structure remain, and the doped amorphous silicon layer 40 and the second metal layer 42 disposed on the source line 14 are completely removed. Next, using photolithography and etching to form an opening on the island structure over the predetermined area of a gate electrode, the second metal layer 42 is separated as source/drain electrodes 42A and 42B and the doped amorphous silicon layer 40 is separated as source/drain diffusion regions 40A and 40B. Thus, the amorphous silicon layer 36 serves as a channel region.
Next, a protection layer 44 is deposited on the entire surface of the transparent substrate 30 and then patterned to form at least a first via 45I, a second via 45II and a third via 45III. The first via 45I and the second via 45II respectively expose parts of the source/drain electrodes 42A and 42B. The third via 45III passes through the first insulating layer 32 to expose a part of the source line 14. Finally, a transparent-conductive ITO layer 46 is deposited and patterned on the entire surface of the transparent substrate 30 to serve as the rectangular-shaped pixel electrode 20. Also, the ITO layer 46 covers the exposed areas of the first via 45I, the second via 45II and the third via 45III to provide electrical connections.
FIG. 2B is a sectional diagram along line A–A′ of FIG. 1 to show a parasitic OLED according to the prior art. When the above-described a-Si:H TFT process is completed, a surface treatment is required before the vapor deposition of a organic/polymer luminescent layer 47 and a cathode metal layer 48. However, the surface treatment normally employs a rinsing process with UV/O3 light or O2 plasma that damages the amorphous silicon layer 36, causing an increased threshold voltage or leakage current. To solve this problem, for the ordinary TFT-LCD process, an annealing treatment additionally applied to the amorphous silicon layer 36 can restore the damaged surface. Nevertheless, for the a-Si:H TFT process, because of the limitation of the subsequent vapor deposition of a organic/polymer luminescent layer 47, it is impossible to use the annealing treatment to restore the damaged surface.
In addition, with regard to the conventional five-mask a-Si:H TFT process, the ITO layer 46 not only serves as the pixel electrode 20, but also serves as an electrical bridge between the second metal layer and the second metal layer or between the second metal layer and the first metal layer. Thus, the ITO layer 46 outside the pixel electrode 20 forms a parasitic OLED area 49 that provides luminescence causing unnecessary power consumption and visual interference.